Boring bars are employed in certain machining operations where it is necessary to machine material in various locations along a common axis, i.e., the rotational axis of the bar. The bar includes a plurality of cutting tools that form a portion of cutting tool cartridges which are removably installed in longitudinal alignment along one side of the bar. The bar is mounted in a machine which rotates the bar about its longitudinal axis, and in some cases, the machine is also capable of displacing the bar along its axis.
One typical application of a boring bar (also sometimes referred to in the art as a "line bar") involves machining the bearings in a block of an internal combustion engine after it has been cast. The block is conveyed along a line to a transfer machine which includes a boring bar which is supported at its opposite ends by bearing assemblies to prevent deflection of the bar during machining operations. With an engine block in place at the machining station, the bar is displaced longitudinally from a standby position to a machining position within the block where the bar is slideably received and supported within support bearings. The engine block bearings are then machined by displacing the block and the rotating boring bar relative to each other. After the cutting operation is complete, the bar slides out of the support bearings and block, back to its standby position to permit transfer of the block to the next machining station. In the case of engine blocks, the blocks may be sucessively transferrred along the line between several identical machining stations with similar boring bars to effect several rough and finish cuts on the block bearings before maching of such bearings is finally complete.
Machining operations using boring bars as described above are performed on various other parts and workpieces, such as transmission housings and the like. The sliding frictional engagement between the support bearings and the outer surfaces of the bar produced during longitudinal displacement of the bar relative to the support bearings results in wear of the surfaces of both the bar and the inner races of the support bearings. Wear is increased by small shavings or chips of machined material which can become trapped between the bar and the inner race; this foreign material causes scoring of the bar and race surfaces when the bar is slid in and out of the race. This problem can become especially pronounced where significant clearance exists between the surface of the bar and inner race. Bar and bearing wear is particularly severe in production line environments where high volume machining requirements are experienced. Bar and bearing wear increases the clearance between the bar and inner races of the support bearings; as this clearance increases, the machining accuracy of the bar diminishes until it becomes necessary to replace the bar and/or bearings. Because boring bars and support bearings are relatively expensive, they are normally "reworked" rather than discarded. Reworking a bar or bearing involves plating the worn surfaces with chrome or the like in order to build these surfaces up to their original dimensions. Reworking is not only expensive in terms of the plating process required to build up the surface, but is also costly in terms of the down-time of the assembly line resulting from change over of the bars.
Another problem associated with prior art boring bars is that of "tool chatter" in which the bar vibrates within the support bearings. Tool chatter results from the fact that excessive clearance between the bar and inner race of the support bearing allows the bar to shift within the inner race at a harmonic frequency which creates vibration. Chatter not only reduces machining accuracy but also increases wear of the bar and bearing surfaces and sets up stresses which can damage either the bar or support bearing.
In the past, boring bars have been made of realtively hard steel, such as 8620 type heat treated steel which may have a hardness of 60 or more as measured on a Rockwell hardness tester. A hard steel is employed in order to reduce wear and/or permit the bars to be plated with a wear-resistant material such as chromium. Chromium plating increases the bar's hardness and stiffness but was desirable because it reduced the surface wear between the bar and support bearings and also allowed worn bars to be replated during rework thereof as previously discussed. Bars constructed of hard steel, especially those which are chrome plated, are brittle and cannot bend or flex to a significant extent without cracking or breaking.
In production line machining operations, severe bending stresses may be created in a bar as a result of inaccurate timing of the longitudinal advancement of the bar relative to the movement of the engine block along the conveyer line; if the bar is advanced prematurely, it may engage a solid part of the block rather than passing through the block, or if the bar is advanced to its machining operation before the next-to-be-machined block has reached the machining station, the bar is disposed within the path of the traveling block and eventually collides with the block. Either of these two eventualities results in extraordinary stresses being applied to the bar which usually cause the bar to crack or break, thus necessitating replacement of the bar and concomitant stopping of the production line to permit change-over of the bars. These same type of bar failures can also result from even slight misalignment between the displacement path of the bar and the position of the support bearing in which the bar is received.
In addition to the problem of bar damage, the extraordinary stresses which result in breaking of the bar pose a serious threat of injury to personnel. These high stresses can propel a broken bar with great force away from the machining station and into the surrounding environment.
Prior art bars are provided with elongate keys which are received within keyways in the inner races of the support bearings. These keys prevent turning of the bar within the inner race; thus, the bar and inner race of the bearing rotate as a unit. The keys normally consist of a plurality of longitudinally spaced, longitudinally aligned, individual key sections which are secured to the bar by screws to permit removal of the keys so that the bar can be machined and plated during rework thereof. The key sections must be individually machined and fitted within machined slots in the bar; this is a labor intensive operation and is therefore costly from a manufacturing standpoint. Additionally, the key sections often become loose from the bar and must be periodically tightened, thus resulting in "down-time" of the production line. In some cases, loose key sections may interefere with proper sliding movement of the bar into the bearing supports which causes binding or jamming that can set up damaging stresses in the bar as previously discussed.